Blend vinegar, salt, sugar, mustard, pickle relish, and capers. Stir in vegetable and olive oil. Set dressing aside.
Shred eggs on a medium-sized grater. Cover and set aside. Clean spinach, removing tough stems. Drain on paper towels and pat dry. Place leaves in a large bowl. Saute bacon until crisp.
Remove bacon and sprinkle over spinach. Add onion and mushrooms to drippings in skillet. Saute until tender. Stir in 1 cup dressing and bring to a boil. Pour over spinach and toss. Garnish with reserved shredded eggs. Serve with additional dressing.
I like my eggs left in whole slices for more flavor. You may wish to grate your hard boiled eggs as directed in this recipe.